All translations agree that it's talking about a snake of some sort; ESV adder NLT deadly snake NASB, NIV viper BST asps but KJV uses here a mystical creature.
And the sucking child shall play on the hole of the asp, and the weaned child shall put his hand on the cockatrice' den. (KJV)
How did they come to this conclusion?
It is true that the species of snake in each case in Isa 11:8 is difficult to identify. Most translators simply guess or rely on a traditional translation.
In the case of the second snake mentioned in Isa 11:8, צֶפַע (tsepha'), most version have "viper"; however this is unlikely, as BDB correctly points out, as vipers do not lay eggs:
צִפְעוֺנִי noun masculine id.; — absolute ׳צ Isaiah 11:8 ("" מָּ֑תֶן), ׳בֵּיצֵי צ Isaiah 59:5; כְּצִפְעֹנִי Proverbs 23:32 (simile; "" נָתָשׁ); plural צִפְעֹנִים Jeremiah 8:17 (app. נְחָשִׁים); identification dubious; TristrNHB 275 ('possibly') daboia xanthina, a venomous viper, but vipers do not lay eggs FurrerBi HWB 2, 1423; Furrer proposes ailurophis vivex.
Thus, the KJV suggestion may be a better possibility, but we cannot be sure.
It may appear to be a 'mystical' creature to us, in this day and age, but the people of Isaiah's era would know what was meant. This can be ascertained from the three times the Hebrew words involved are used by Isaiah, with Jeremiah agreeing in his one use.
There are two Hebrew words; tsepha means cockatrice, basilisk, adder and viper.
The other Hebrew word is tsiphoni and simply means cockatrice.
Tsepha is only used once in the Hebrew scriptures, by Isaiah, in 14:29, where he says:
"Rejoice not thou, whole Palestina, because the rod of him that smote thee is broken: for out of the serpent's root shall come forth a cockatrice, and his fruit shall be a fiery flying serpent." (A.V.)
This is in the context of the coming desolation of Babylon, Israel's triumph in the fall of Babylon's king, and Palestina being threatened with complete dissolution.
Here are the three uses of the other word, tsiphoni, all from the A.V.
"And the sucking child shall play on the hole of the asp, and the weaned child shall put his hand on the cockatrice' den." Isaiah 11:8
"They hatch cockatrice' eggs, and weave the spider's web: he that eateth of their eggs dieth, and that which is crushed breaketh out into a viper." Isaiah 59:5
"For, behold, I will send serpents, cockatrices, among you, which will not be charmed, and they shall bite you, saith the Lord." Jeremiah 8:17
It seems clear that, whatever we think of the word 'cockatrice', the two prophets used it in tandem with speaking of snakes, almost as synonyms. Our lack of understand does not detract from the force of the prophetic statements. We surely all get the gist of their meaning? Thus, my answer is that some kind of snake is meant, known to them, if not to us. There's nothing mystical about it.
